ne of the earliest extensive series of
experiments was made under the writer's direction in 1881, with a large
number of models, the primary object being to determine what value there
was in a few of the various twists which inventive ingenuity can give to
a screw blade. The results led the experimenters to the conclusion that
in free water such twists and curves are valueless as serving to augment
efficiency. The experiments were then carried further with a view to
determine quantitative moduli for the resistance of screws with
different ratios of pitch to diameter, or "pitch ratios," and afterward
with different ratios of surface to the area of the circle described by
the tips of the blades, or "surface ratios." As these results have to
some extent been analyzed and published, no further reference need be
made to them now.

In 1886, Mr. R.E. Froude published in the Transactions of the
Institution of Naval Architects the deductions drawn from an extensive
series of trials made with four models of similar form and equal
diameter, but having different pitch ratios. Mr. S.W. Barnaby has
published some of the results of experiments made under the direction of
Mr. J.I. Thornycroft; and in his paper read before the Institution of
Civil Engineers in 1890 he has also put Mr. R.E. Froude's results into a
shape more suitable for comparison with practice. Nor ought Mr. G.A.
Calvert's carefully planned experiments to pass unnoticed, of which an
account was given in the Transactions of the Institution of Naval
Architects in 1887. These experiments were made on rectangular bodies
with sections of propeller blade form, moved through the water at
various velocities in straight lines, in directions oblique to their
plane faces; and from their results an estimate was formed of the
resistance of a screw.

One of the most important results deduced from experiments on model
screws is that they appear to have practically equal efficiencies
throughout a wide range both in pitch ratio and in surface ratio; so
that great latitude is left to the designer in regard to the form of the
propeller. Another important feature is that, although these experiments
are not a direct guide to the selection of the most efficient propeller
for a particular ship, they supply the means of analyzing the
performances of screws fitted to vessels, and of thus indirectly
determining what are likely to be the best dimensions of screw for a
vessel of a class whose r